Boilers that are mainly used in ordinary homes to heat and supply hot water are categorized into gas boilers and oil boilers according to the type of fuel used. The gas boilers sometimes use liquefied petroleum gas (LPG) as fuel, but mostly use liquefied natural gas (LNG), which is a clean fuel capable of minimizing air pollution because it contains almost none of a sulfur (S) component, compared to light oil or kerosene used for the oil boilers.
The boilers can be classified, according to the type of heat exchangers, into condensing boilers and non-condensing boilers. In the boilers that generally use gas or oil as fuel, studies for increasing heat efficiency have been multilaterally made. As a result, a condensing boiler, which makes use of the latent heat of condensation of water vapor (H2O), a product of combustion, contained in an exhaust gas, has been recently developed.
A condensing heat exchanger functions to reduce the loss of radiant heat using a principle of recovering the latent heat of condensation, and is provided with a sensible heat exchanger that directly heats water using combustion heat of a burner, as well as a latent heat exchanger that heats water using the latent heat of exhaust gas passing through the sensible heat exchanger. Thus, the condensing heat exchanger can lower the temperature of exhaust gas below a dew point temperature to absorb the condensation latent heat from the exhaust gas, thereby improving heat efficiency.
In the condensing heat exchanger, however, corrosion takes place due to acidic liquid of about pH 2 to 4 which is produced by the reaction of moisture produced during condensation with sulfur oxides (in the case of the oil) or nitrogen oxides (in the case of the gas) in the exhaust gas, as well as combustion heat. Inhibition of this corrosion has always emerged as a problem. In most condensing heat exchangers for solving this problem, castings formed of special stainless steel, aluminum alloy, etc. have been used for heat exchanging fins, heat exchanging tubes, and so on, which constitute the heat exchanger. However, these condensing heat exchangers are difficult to manufacture, and are large in size and heavy in weight, so that they incur expensive production costs. Consequently, these factors act as considerable restrictions in realizing the simple assembly of the heat exchangers.
Compared to the heat exchanger of the condensing boiler, the non-condensing boiler is provided with a sensible-heat heat exchanger alone, so that it incurs relatively low production costs, is easy to assemble, and can be made compact. However, it is absolutely necessary to strictly prevent water vapor from condensing.
Unlike the condensing boiler based on the condensation of water vapor in the exhaust gas, the heat exchanger of an ordinary boiler is typically made of a material such as copper (Cu), ordinary stainless steel, or the like. When condensed water is brought into contact with the heat exchanger made of this material, serious corrosion takes place, which decreases heat exchange efficiency, and causes the function of the heat exchanger to be lost in the long run.
Thus, a method for operating the non-condensing boiler at maximum heat efficiency and without condensation is required.